1. Field of the Invention
The present invention relates to an impedance-variation-type humidity sensor for use in an exhaust gas exhausted from, for example, a smoke discharge apparatus, an exhaust duct, or an exhaust-gas purifying apparatus of an internal combustion engine, as well as to a method of using the humidity sensor in the exhaust gas.
2. Description of the Related Art
Conventionally, in order to detect humidity with high accuracy over a long period of time, the impedance-variation-type or rather resistance-variation-type humidity sensors commercially available for industrial use are periodically subjected to heat-applied cleaning so as to burn out fouling substances such as dust, deposits, carbon, coke (insufficiently fired fuel) and crystal water deposited on a humidity-sensitive element thereof.
For example, Patent Document 1 proposes a method of recovering the measuring accuracy of a humidity sensor when the measuring accuracy is lowered as a result of adhesion of tar from cigarette smoke to a humidity-sensitive element held in an ambient atmosphere. Specifically, a ceramic heater of the humidity sensor is electrically energized to heat the humidity-sensitive element to a temperature of about 500° C. for one minute or so, thereby cleaning off or burning out adhered fouling substances from the surface of the humidity-sensitive element so as to refresh the sensor element and recover the impedance of the sensor element degraded by the fouling substances formed on the surface thereof.
With regard to the resistance-variation-type humidity sensor for use in an exhaust gas atmosphere of an internal combustion engine, Patent Document 2, discloses a method of avoiding a condition under which water condensation or coking is prone to occur, by actuating a heater in accordance with an operating condition (e.g., duration of idling) of the internal combustion engine.
[Patent Document 1]
U.S. Pat. No. 4,801,211 to Yagi et al.
[Patent Document 2]
United States Patent Application Publication US 2001/0025484 A1 to Ueno et al.
3. Problems to be Solved by the Invention:
However, the inventors have found that heat-applied cleaning of the humidity sensor element under a temperature of about 500° C. as proposed in Patent Document 1 above fails to completely remove fouling substances adhered on the humidity-sensitive element of the humidity sensor when subject to an exhaust gas of an internal combustion engine, resulting in a failure to detect humidity with high accuracy and durability.
With respect to a humidity sensor applied to an internal combustion engine, the technique as described in Patent Document 2 fails to sufficiently clean the humidity sensor. This is because exhaust gas temperature is generally low at a position located under the floor of a vehicle where the humidity sensor is mounted.
However, even though the heater is energized when the engine continues to idle, the heating temperature is high enough only to evaporate condensed water adhered on the humidity sensor. In other words, the heating temperature is not sufficiently high to completely remove other fouling substances from the humidity-sensitive element, resulting in a failure to detect humidity with high accuracy over a long period of time.